Generally, in LED lighting applications, AC-DC power conversion sources are often applied as power supplies for driving the LED. Typically, two types of AC-DC power conversion topologies, either isolated or non-isolated can be used.
FIG. 1 illustrates an isolated topology application 50A for LED driving. As shown in FIG. 1, an AC voltage signal VAC is converted to a DC voltage signal VDC by a rectification circuit 51. Then the DC voltage signal VDC is further converted to a suitable DC voltage signal for driving LEDs 53 by a flyback voltage converter 52 which comprises a transformer. The transformer of the flyback voltage converter 52 isolates a primary circuit and a secondary circuit in this isolated topology application. In one embodiment, the LEDs 53 may comprise a single LED. In one embodiment, the LEDs 53 may comprise a LED string. In one embodiment, the LEDs 53 may comprise a plurality of LED strings. In a LED driving application, the most important point is that a constant average current ILED flowing through the LEDs 53 are needed to be achieved by a constant current control method. Generally, the average current ILED in the secondary circuit is sensed as a feedback signal provided to a control circuit for regulating the constancy of the average current ILED. The control circuit is configured to receive the feedback signal and coupled to a switch 521 in the primary circuit for switching the switch 521 on and off. However, since the primary circuit and the secondary circuit are isolated in this application, the average current ILED in the secondary circuit can not be coupled to the control circuit directly. An expensive optocoupler or other suitable devices should be applied resulting in a high cost.
FIG. 2 illustrates a non-isolated topology application 50B. As shown in FIG. 2, comparing to the isolated topology application 50A, the transformer is omitted in the non-isolated topology application 50B. Therefore, LEDs 53 are coupled to the rectification circuit 51 for receiving a DC voltage signal VDC directly. In this situation, sensing an average current ILED flowing through the LEDs 53 is infeasible due to the high DC voltage signal VDC across the LEDs 53.
Accordingly, a cost-effective and feasible LED driving circuit, a control circuit and associated current sensing circuit for LED lighting applications are desired.